A conventional method for producing metal flakes involves the use of a ball mill in which starting metal particles are tumbled together to form metal flakes. One such method is described in U.S. Pat. No. 4,469,282, which describes the production of metal flake in a wet mill grinding operation. Indeed, the primary commercial method with which metal flake is made employs a wet ball milling technique. This technique creates lamellar metal particles in an aqueous phase of, for example, an organic liquid. However the wet method requires use of specialized equipment to handle its volatile solvents and also creates a hazardous waste stream, which may lead to increased production costs.
On the other hand, metal flake can be produced using a dry method, wherein flakes are produced in air or gas and wherein the use of volatile solvents can be avoided. The dry method is time dependent, and particles are milled for an optimum time, as determined by one of skill in the art. According to U.S. Pat. No. 4,115,107, care must be taken that dry milling not continue for too long a period because continued pounding of the particles by the milling media may result in cold welding or irreversible agglomeration. The dry method has not been widely adopted.
Zinc particles are used in various types of coating compositions. Such particles exist in three forms: powder, dust and flake. Zinc in the form of a flake has significantly greater surface area and reactivity than dust or powder and therefore has attractive commercial applications. Zinc flake has been produced by various methods, and zinc-containing coating compositions are used to protect ferrous-containing surfaces, among others.
For example, U.S. Pat. No. 3,389,105 describes a procedure for making flake metal powder for incorporation into metallic pigments in inks, paints and the like, i.e. in dispersion in a suitable film-forming vehicle to provide a protective or decorative surface-coating composition.
Zinc flakes may be mixed with a binder, initially in a liquid composition but subsequently in a solid composition after curing and/or drying. The binder creates a matrix that incorporates the particles in a coherent mass that is bonded to the surface of a substrate. For use in a paint, the zinc flakes and binder are initially mixed with a liquid paint vehicle, which serves to allow the mixing and applying of the paint to the substrate and thereafter dries and evaporates to leave a paint coating mixture of particles and binder adhered to the substrate.
The coating of surfaces, such as ferrous-containing surfaces, with zinc-containing coating compositions has several purposes. One purpose is for pure esthetics, but mainly zinc-containing coating compositions function to prevent rust and corrosion. Indeed, preventing or inhibiting rust and corrosion can save hundreds of millions of dollars annually for industrial and consumer users.
Whether used as a coating for a wrought iron deck railing at a residence or apartment building or for coating the metal surfaces of a bridge or similar structure, the extent to which corrosion and rusting is prevented can have a significant impact on the useful life of a structure. Thus, intra-coating cohesive properties, meaning those measuring the strength of the internal bond of the topcoat rather than the bond between the topcoat and the substrate, are important, as are the adhesive qualities of a coating applied to a substrate.
It is known to mix a small portion of metal flake as an additive with paint and other coating materials to prolong the life of a surface covered by the improved coating material. Moreover, lamellar particles enhance the pot life of liquid coatings by improving settling characteristics, and they also lengthen the service life of a dry film by increasing cathodic protection and by creating an improved barrier to environmental exposure. However, there is a need in the art for a milling process that will produce a metal flake additive having improved adhesive and cohesive performance characteristics and displaying the requisite characteristics to allow for inclusion in certain modern binder chemistries so that metal flakes may be used in conjunction with advanced modern top coating techniques.
The present disclosure provides a metal flake, and a method for production thereof, having improved adhesive and cohesive properties, which facilitate the production of coating compositions exhibiting improved adhesive and cohesive qualities. These and other advantages of the present disclosure will be apparent from the description set forth below.